Due to the rapid development of digital technology since the 1950's, the development of analog devices has been essentially squeezed out to the periphery of data acquisition equipment only. It could be argued that the conversion to digital technology is justified by the flexibility, universality, and low cost of modern integrated circuits. However, it usually comes at the price of high complexity of both hardware and software implementations. The added complexity of digital devices stems from the fact that all operations must be reduced to the elemental manipulation of binary quantities using primitive logic gates. Therefore, even such basic operations as integration and differentiation of functions require a very large number of such gates and/or sequential processing of discrete numbers representing the function sampled at many points. The necessity to perform a very large number of elemental operations limits the ability of digital systems to operate in real time and often leads to substantial dead time in the instruments. On the other hand, the same operations can be performed instantly in an analog device by passing the signal representing the function through a simple RC circuit. Further, all digital operations require external power input, while many operations in analog devices can be performed by passive components. Thus analog devices usually consume much less energy, and are more suitable for operation in autonomous conditions, such as mobile communication, space missions, prosthetic devices, etc.
It is widely recognized (see, for example, Paul and Hüper (1993)) that the main obstacle to robust and efficient analog systems often lies in the lack of appropriate analog definitions and the absence of differential equations corresponding to known digital operations. When proper definitions and differential equations are available, analog devices routinely outperform corresponding digital systems, especially in nonlinear signal processing (Paul and Hüper, 1993). However, there are many signal processing tasks for which digital algorithms are well known, but corresponding analog operations are hard to reproduce. One example, which is widely recognized to fall within this category, is related to the use of signal processing techniques based on order statistics1. 1See, for example, Arnold et al. (1992) for the definitions and theory of order statistics.
Order statistic (or rank) filters are gaining wide recognition for their ability to provide robust estimates of signal properties and are becoming the filters of choice for applications ranging from epileptic seizure detection (Osorio et al., 1998) to image processing (Kim and Yaroslavsky, 1986). However, since such filters work by sorting, or ordering, a set of measurements their implementation has been constrained to the digital domain. As pointed out by some authors (Paul and Hüper, 1993, for example), the major problem in analog rank processing is the lack of an appropriate differential equation for ‘analog sorting’. There have been several attempts to implement such sorting and to build continuous-time rank filters without using delay lines and/or clock circuits. Examples of these efforts include optical rank filters (Ochoa et al., 1987), analog sorting networks (Paul and Hüper, 1993; Opris, 1996), and analog rank selectors based on minimization of a non-linear objective function (Urahama and Nagao, 1995). However, the term ‘analog’ is often perceived as only ‘continuous-time’, and thus these efforts fall short of considering the threshold continuity, which is necessary for a truly analog representation of differential sorting operators. Even though Ferreira (2000, 2001) extensively discusses threshold distributions, these distributions are only piecewise-continuous and thus do not allow straightforward introduction of differential operations with respect to threshold.
Nevertheless, fuelled by the need for robust filters that can operate in real time and on a low energy budget, analog implementation of traditionally digital operations has recently gained in popularity aided by the rapid progress in analog Very Large Scale Integration (VLSI) technology (Mead, 1989; Murthy and Swamy, 1992; Kinget and Steyaert, 1997; Lee and Jen, 1993). However, current efforts to implement digital signal processing methods in analog devices still employ an essentially digital philosophy. That is, a continuous signal is passed through a delay line which samples the signal at discrete time intervals. Then the individual samples are processed by a cascade of analog devices that mimic elemental digital operations (Vlassis et al., 2000). Such an approach fails to exploit the main strength of analog processing, which is the ability to perform complex operations in a single step without employing the ‘divide and conquer’ paradigm of the digital approach.
Perhaps the most common digital waveform device is the analog-to-digital converter (ADC). Among the salient characteristics of ADCs are their sampling frequency, measurement resolution, power dissipation, and system complexity. Sampling frequency is typically dictated by the signal of interest and/or the requirements of the application. As the frequency content of the signal of interest increases and the sampling frequency increases, resolution decreases both in terms of the absolute number of bits available in an ADC and in terms of the effective number of bits (ENOB), or accuracy, of the measurement. Power needs typically increase with increasing sampling frequency. The system complexity is increased if continuous monitoring of an input signal is required (real-time operation). As an example, high-end oscilloscopes can capture fast transient events, but are limited by record length (the number of samples that can be acquired) and dead time (the time required to process, store, or display the samples and then reset for more data acquisition). These limitations affect any data acquisition system in that, as the sampling frequency increases, resources will ultimately be limited at some point in the processing chain. In addition, the higher the acquisition speed, the more negative effects such as clock crosstalk, jitter, and synchronization issues combine to reduce system performance.
It is highly desirable to extract signal characteristics or preprocess data prior to digitization so that the requirements on the ADCs are reduced and higher quality data can be obtained. In the past, one common technique was to use an analog memory to sample a fast signal and then the analog memory would be clocked out at a low speed and digitized with a moderately high resolution ADC. While this technique works, it suffers from significant degradation due to clock feedthrough, non-linear effects of the analog memories chosen, and limited record length. Another technique used is to multiplex a high-speed signal to a number of lower speed but higher resolution ADCs using an interleaved clock. Again, the technique works but never realizes the best performance of a single channel due to the high clock noise and inevitable differences in processing channels.
The introduction of the Analysis of VAriables Through Analog Representation (AVATAR) methodology (see Nikitin and Davidchack (2003a) and U.S. patent application Ser. No. 09/921,524, which are incorporated herein by reference in their entirety) is aimed to address many aspects of modern data acquisition and signal processing tasks by offering solutions that combine the benefits of both digital and analog technology, without the drawbacks of high cost, high complexity, high power consumption, and low reliability. The AVATAR methodology is based on the development of a new mathematical formalism, which takes into consideration the limited precision and inertial properties of real physical measurement systems. Using this formalism, many problems of signal analysis can be expressed in a content-sentient form suitable for analog implementation. Specific devices for a wide variety of signal processing tasks can be built from a few universal processing units. Thus, unlike traditional analog solutions, AVATAR offers a highly modular approach to system design. Most practical applications of AVATAR, however, are far from obvious, and their development requires technical solutions unavailable in the prior art. For example, AVATAR introduces the definitions of analog filters and selectors. Nonetheless, the practical implementations of these filters offered by AVATAR are often unstable and suffer from either lack of accuracy or lack of convergence speed, and thus are unsuitable for real-time processing of nonstationary signals. Another limitation of AVATAR lies in the definition of the threshold filter. Namely, a threshold filter in AVATAR depends only on the difference between the displacement and the input variables, and is expressed as a scalar function of only the displacement variable, which limits the scope of applicability of AVATAR. As another example, the analog counting in AVATAR is introduced through modulated density, and thus the instantaneous counting rate is expressed as a product of a rectified time derivative of the signal and the output of a probe. Even though this definition theoretically allows counting without dead time effect, its practical implementations are cumbersome and inefficient.